This dissertation proposes to assess the accuracy of endogenous and exogenous infant mortality proxies. These proxies include the neonatal, postneonatal, 0-3 day, 0-18 day, and 4-364 day mortality rates. Various proxies and their referents will be examined descriptively and analytically. Further, to determine their generalizability, the best approximations of endogenous and exogenous infant mortality rates among various countries will be examined, including the United States. Last, the referents themselves will be analyzed over time in the United States. Their trends will be interpreted through the epidemiological transition theory. The data include a matched set of New Mexico infant death and birth records, records for the United States, and records for other countries. The methods include epidemiological techniques, for example, sensitivity and specificity indexes; descriptive statistics, for example, differences between rates; regression analyses, for example, hyperbolic curve fitting and binary variable analysis; and log-linear analysis. Preliminary investigation suggests that the two most widely used proxies of endogenous and exogenous mortality, namely, neonatal and postneonatal mortality, are not the best proxies available and show discrepant results from their referents.